“Allowing these drugs to be purchased without a prescription and without any advice or restrictions sends a message to the public that they must be safe,” Gunnar H. Gislason, author of the study and professor of cardiology, said in a news release. “The findings are a stark reminder that NSAIDs are not harmless.”

To come up with the results, researchers used the Danish Cardiac Arrest Registry to collect the medical history of nearly 30,000 patients who had suffered a heart attack outside of the hospital from 2000 to 2010.

They also examined all NSAID prescriptions filled at Danish pharmacies since 1995.

According to Time, ibuprofen and naproxen, both available over the counter in the United States as Advil and Aleve, require prescriptions in Denmark.

The only NSAID sold without a prescription, Time reported, is ibuprofen in 200mg, the same as a regular-strength Advil in the U.S.

Both diclofenac, which requires a prescription in both Denmark and the U.S., and ibuprofen were found to be the most commonly used NSAIDs in heart attack cases, the study found.